A positive photoresist composition generally contains an alkali-soluble resin and a photosensitive substance of a naphthoquinonediazide compound. For example, novolak type phenol resins/naphthoquinonediazide substituent compounds are described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470; and examples of the most typical compositions include cresol-formaldehyde novolak resins/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfonates as described in L. F. Thompson, Introduction to Microlithography (published by ACS Publishing Co., No. 219, pages 112 to 121).
Novolak resins used as a binder are soluble in an aqueous alkaline solution but do not swell therein and are highly resistant to plasma etching when the image pattern is used as an etching mask, such that novolak resins are especially advantageous. On the other hand, naphthoquinonediazide compounds when used alone as a photosensitive substance act as a dissolution inhibitor to lower the alkali solubility of a novolak resin, but are decomposed by light irradiation to release an alkali-soluble substance which elevates the alkali solubility of the novolak resin. Due to the light dependent variation of the properties of these compounds, naphthoquinonediazide compounds are especially useful as a photosensitive substance component in a positive photoresist composition.
Hitherto, various resists have been proposed in order to obtain a high resolving power needed for processing of ultrafine patterns. One such photoresist composition contains a complete ester as a photosensitive component, wherein all the hydroxyl groups of a polyhydroxy compound have been esterified by elevating the esterification degree.
JP-A-61-45240 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") describes the use of 1,2-naphthoquinonediazidotrisulfonates of trihydroxybenzenes. JP-A-61-118744 and JP-A-62-280737 describes the use of 1,2-naphthoquinonediazidotetrasulfonates of tetrahydroxybenzophenones. Resists containing such 1,2-naphthoquinonediazidosulfonates having an elevated esterification degree as a photosensitive component have a high resolving power but exhibit low sensitivity, such that manufacturing efficiency is decreased. The sensitivity could be enhanced by reducing the proportion of the photosensitive component in the resist, or by addition of a sensitizing agent to the resist, or by elevating the alkali activity of the developer for processing the exposed resist. In such cases, however, the resolving power of the resist is substantially degraded such that the resist is no longer useful for forming ultrafine patterns.
Elevation of the esterification degree alone therefore does not provide both high resolving power and high sensitivity. "Elevation of esterification degree" as used herein means an increase in the proportion of the complete ester form of the light-sensitive component.
It is well known by those skilled in the art that a 1,2-naphthoquinonediazidosulfonate of a polyhydroxy compound which is conveniently used in a positive photoresist composition is not a single compound but a mixture of many isomers.
For example, U.S. Pat. Nos. 4,409,314 and 4,439,516 and JP-A-62-280737 describe that 1,2-naphthoquinonediazidosulfonates of dihydroxybenzophenone or trihydroxybenzophenone are in the form of a mixture of the esters.
U.S. Pat. No. 3,148,983 describes a 1,2-naphthoquinonediazidotrisulfonate of tetrahydroxybenzophenone, which description is not considered to be completely technically accurate. N. Ogata et al found that the ester is also in the form of a mixture of plural isomers thereof, as disclosed in Functional Materials, pages 43 to 46 (December, 1987).
The present inventors isolated and purified particular isomers other than the complete ester in various photosensitive substances and evaluated and investigated the resist capacity of the isomers. As a result, the present inventors have discovered certain isomers having extremely high sensitivity and high resolving power which could not be expected from conventional resists containing 1,2-naphthoquinonediazidosulfonates of polyhydroxy compounds in the form of various mixtures of the esters.
Furthermore, the present inventors have discovered a novel method of efficiently producing these isomers.